Objectives: Anaerobic bacteria are relatively common pulmonary pathogens. Anaerobic infections are very often polymicrobial, including also microaerophilic and facultative anaerobes. Some studies have shown that patients with underlying carcinoma have high incidence of Gram-negative aerobic pathogens. The purpose of this study was to determine the frequency of different anaerobic bacterial species and their susceptibility to antimicrobial agents. In patients with underlying carcinoma we wanted to determine if Gram-negative aerobic bacteria are frequently present together with anaerobes, which could have implications on empirical treatment of these infections.

Methods: This study was conducted during year 2005 in Department for Microbiology, University Hospital for Lung Diseases ``Jordanovac''. Anaerobic bacteria were isolated in patients with clinical and radiographic signs of pulmonary infection from specimens obtained mostly at fiber-optic bronchoscopy, either by bronchoscopic aspiration or by bronchoalveolar lavage (BAL). Antimicrobial susceptibility was tested with E-tests in order to determine minimal inhibitory concentrations (MIC) according to NCCLS (CLSI) standards.

Results: Total of 108 anaerobes was isolated in 27 female and 64 male patients. The most frequent isolates were Gram-positive non-spore-forming bacilli (30.56%). Anaerobes alone were detected in 61 (67.03%), and anaerobic and aerobic bacteria together were observed in 30 (32.97%) cases. An average of 1.65 bacterial species per patient was obtained. Underlying carcinoma was present in 24 (26.37%) patients. In 20 (83.3%) of these patients anaerobes were present alone. The level of resistance to metronidazole, ampicillin and clindamycin was 59.3%, 8.3% and 6.5%, respectively. To imipenem and ko-amoxiclav there were no resistant strains.

Conclusion: Anaerobic bacteria are found more often in male patients with pulmonary infection. The most frequent anaerobes in our patients were Gram-positive non-spore-forming bacilli. In most of our patients multiple isolates were found. In contrast to the findings of some studies in patients with underlying carcinoma, anaerobes were more frequently present alone and Gram-negative aerobic bacteria were uncommon. High percent of resistance to metronidazole indicates that clindamycin and b-lactams are better choice when anaerobic infection is suspected.